I've just downloaded and read the Schedule for the International Faceting challenges. I applaud the efforts to provide guidance for competitors by way of pertinent rules in the Schedule. Especially as the Schedule does not state the rules in the Competitor and Judging Manual Lapidary and Allied Competitions published by AFLACA apply.
Given this is an International competition, it would perhaps be unfair on international competitors for those rules to apply given they probably don't have ready access to them.
But I'm a little confused by rule E5 which requires the girdle thickness to be 5% with a tolerance of 0.01mm above or below the calculated thickness. Unfortunately my graticule only has 0.1mm increments so I'm not sure how I can be sure of complying with the requirement if I enter.
This would be the toughest specified size in any competition I've seen, but after all it is an International Challenge. The US Facetors Guild specifies the size of the girdle for their competitions and allows the competition committee to specify the girdle thickness. The samples I looked at all had specified the girdle size as 0.3mm + or - 0.1mm in all divisions except for Novice division (which specified a girdle thickness of 0.5mm +/- 0.3mm last year).
However Rule E5 doesn't say whether the 5% is 5% of W or 5% of the height of the finished stone, or the increments in which marks are lost.
Rule E5 also conflicts with the judging sheets which provide for a maximum girdle width.
Would you please post a note on the same web page as the competition schedule and designs for the IFC competition are downloaded from clarifying rule E5 and the girdle requirements for the competition.
Gordon,
You are correct. It should be 0.1mm
Girdle thickness is always a percentage of the TOTAL HEIGHT.
Why would it be a percentage of the Width?
Are you confused with the "W" aspect when calculating P/W & C/W? added together then multiplied by 5% to obtain the girdle thickness?
President Paul ,
You ask why would the width be used when calculating the girdle size .
This subject was raised at a recent meeting of the Hunter Valley group and no-one knew why the girdle width had to be calculated from the Stone's height .
The width of the stone is known at the time of transfer from pavilion to crown but the actual height is not known until the stone is finished and taken off the dop . A theoretical height can be calculated from the Gemcad data but in the event that some angles may have been changed during cutting this is not reliable and it could happen that with a fine degree of tolerance in the allowable girdle thickness the stone may fall outside these limits if some angles are changed.
For the benefit of facetors such as our group a reason why the height is used rather than the width will be welcome..
John Maine
As I understand it the girdle thickness is a percentage of the height because it produces a consistent result. For example the girdle of an oval or rectangle would differ depending on which dimension you took.
I have drafting CAD on my computer and I drew some side views of comp. stones. You’re going to have to make some significant angle changes on an average size stone to impact the required girdle thickness.
Altering the pavilion mains on a 16mm diameter standard brilliant from 43deg to 40deg only alters the height by .75mm. 5 percent of that is 0.0375mm which is a third of the 0.1 tolerance.
Guys,
I guess the word "width" is confusing when we say "girdle width".
I think in terms of "Thickness". The only reference to width is when calculating P/W & C/W from the data on the gemcad diagram.
Take a read of the attachment which hopefully explains it all.
Paul
Frank ,
Thanks for your information. Your example makes it clearer that it is unlikely for a facetor to measure the height of the finished stone and find the girdle thickness is outside the allowable limits , even if angles have had to be changed because of unforeseen problems when such things as small bubbles , inclusions etc. appear.
Well, that could be a problem. If it was a competition stone and it ended up under the 5% but not less than 1% it should be OK. You have a big margin to play with between 1% and 5%. It doesn't have to be exactly 5% it can be under, I would work to around 2-3% to be safe.
It's only when you leave it at over 5% you will loose points or it is too thin.
If it ends up really thin, say around the 1% the judge won't be able to see other features like scratches, uniformity, flatness, that's why you get penalised if it is too thin. Plus that makes it harder to set in a jewellery piece, it would probably break off at the claw.
Is that easier to understand.
If the girdle thickness is 5% of the height of the stone then that’s the tolerance.
Below is a copy of the IFC rules regarding girdle thickness.
E5. Girdle Thickness will be calculated as a comparison of the Calculated Girdle Thickness against
Actual Girdle Thickness of 5% as measured by the Judge using a Graticule Loupe. There will be
an allowed tolerance of ±0.01mm above or below the calculated thickness.
So if the stone is 8mm high then 5% is 0.4mm and that’s the girdle thickness + or – 0.01mm, which is impossible to measure, so you just have to do the best you can.
Working to 2-3% of the stone height will loose you all points for the girdle, or any other percentage for that matter.
I enter both the US and UK comps. Both set their girdle thickness at 0.3mm. In my opinion it would be a lot simpler for both judges and entrants in the AU comps if it was the same.
Hi Frank,
I posted the Dorothy Dix that started this thread because I interpreted the rule the same way but wasn't satisfied as the wording was still ambiguous. The rule is also inconsistent with the judging sheets supplied with the rules.
I have since found out the rule was intended to operate to specify the maximum height of the girdle, which is how they are judged in Australia.
I've suggested an alternative wording for use with future comps but it is too late to change the rule for this comp.
The +/- 0.01mm was a mistake the tolerence should be 0.1mm. Paul tells me he has fixed that in the copy of the schedule on the AFG website.
About the only thing to be done with the rule is seize on the inconsistency with the judging sheets and post an explanation of how it is intended to be interpreted on the page where you download the schedule.
I think this would benefit US cutters as they seem to be used to a set sized 0.3mm girdle width +/- 0.1mm for competition stones.
Regards
Gordon
It's a typo. The tolerances are 0.1mm.
The schedule has been corrected.
"Girdle Thickness will be calculated as a comparison of the Calculated Girdle Thickness against Actual Girdle Thickness of 5% as measured by the Judge using a Graticule Loupe. There will be an allowed tolerance of ±0.1mm above or below the calculated thickness."
This is how it's done.....The judge will measure the Total Height, (Culet to Table) calculate what 5% is, using a calculator, write that on the sheet, then measure the actual thickness with a graticule loupe, write that on the sheet, then compare the two. If the result is more than the calculated amount and allowing for the tolerance deduct points accordingly. e.g. calculated thickness is .4mm, actual thickness is .6mm minus the .1mm tolerance then the girdle is too thick by .1mm and will loose points. That's how judges do it here in Australia.
What other countries do is different, this comp will be judged as above.
My previous post has an article attached explaining how you work out the girdle thickness before you actually cut it.
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